1. Field of the Invention
The present invention generally relates to a technique for mounting components of devices such as semiconductor devices on a circuit board and, more particularly, to a method and a device for manufacturing a circuit board to allow for effective use of lead-free solder from a viewpoint of environmental protection recently in public attention. Tin-lead eutectic crystal solder has been conventionally used in general because of its overall supremacy in cost, quality, handling facility and other respects.
In recent years, however, noxiousness of lead has been pointed out, and efforts are being made to use as little lead as possible. On the other hand, some components which are joined to a circuit board still contain lead solder. Thus, until lead-containing solder is completely out of use, an effective circuit board mounting technique which can be applied to both lead solder and lead-free solder is required, while use of lead-free solder is being promoted.
2. Description of the Related Art
Conventionally, tin-lead eutectic crystal solder has been used as a material to electrically and mechanically join a circuit board and components such as semiconductor components, capacitors and resistors. On the other hand, terminals of the components themselves are either soldered or plated beforehand with tin-lead, the same elements as the joining material applied on the circuit board, to increase wettability and adhesion of the components to the circuit board. Here, a conventional mounting process to join the circuit board and the components comprises the steps of: adapting a technique of screen stencil to apply tin-lead solder to the circuit board through a screen mask all at one time; placing the components at certain locations of the circuit board; and joining the circuit board and the components by heating. Tin-lead solder as used in the above-mentioned process has been satisfactory in terms of cost, quality, durability, processing facility and other respects.
Recently, however, almost all industries have begun to avoid using lead to protect the global environment. Accordingly, in the circuit board mounting industry area where tin-lead solder has been widely used as a joining material, lead-free solder has begun to be developed.
Hence, varieties of lead-free solder are being proposed to replace conventional tin-lead solder. Generally, tin-based solder, which is technically established, is dominant, including tin-bismuth solder, tin-silver solder, tin-copper solder and tin-zinc solder. Adding a third, fourth or more element to each of these varieties of lead-free solder can create another variety of lead-free solder. Moreover, changing the elemental ratio of the above-mentioned varieties of lead-free solder can create even more varieties of lead-free solder.
In practice, manufacturers who mount components on a circuit board use their own kind of lead-free solder as a joining material depending on temperature of mounting, strength of adhesion, cost and purpose of use. Some of the manufacturers mount on a circuit board components made by a plurality of manufacturers. That means that several types of components are mounted on a circuit board. As mentioned above, to increase wettability and adhesion of components to a circuit board, terminals of the components themselves are either soldered or plated beforehand with either tin-lead solder or lead-free solder of a number of varieties. To effectively increase the wettability and adhesion of components to a circuit board, the joining material applied on the circuit board should be the same solder as that soldered or plated beforehand on the terminals of the components. However, in practice, manufacturers who mount components on a circuit board are applying one kind of lead-free solder to the circuit board as the joining material, because it is ineffective to apply different kinds of solder to the circuit board for each of the components.
However, some kinds of solder applied as a joining material on a circuit board do not match other kinds of solder or plating formed beforehand on the terminals of components, which reduces the strength of adhesion between the circuit board and the components. Facing this problem, manufacturers have to stop applying simply one kind of lead-free solder on a circuit board, but instead manually apply different kinds of solder to the circuit board for each of the mismatching components to secure a sufficient strength of adhesion.
However, a dispensing method, which is often employed as the manual applying method, requires an expert to be in operations for a relatively long time. Other problems are that supply of solder varies widely and that some configurations of terminals of components do not allow the manual applying method.
Moreover, mounted components have drastically been miniaturized, multi-pinned and made highly dense, which narrows a joining area and reduces the strength of adhesion. To make up for the reduced strength of adhesion, solder which provides a high strength of adhesion is required. On the other hand, for large-sized components, a low temperature solder is sought to be put in use, because a temperature of terminals of the large-sized components is hard to raise. These trends, together with the above-mentioned varieties of lead-free solder, increase varieties of solder used in mounting components.
It is a general object of the present invention to provide an improved and useful method for manufacturing a circuit board in which method the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a method for manufacturing a circuit board which causes no problems when a plurality of types of components provided beforehand with respectively different kinds of joining material are mounted.
Another object of the present invention is to provide a manufacturing device for embodying the method in the first object.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a method of manufacturing a circuit board on which are to be mounted a plurality of types of components having terminals on which a joining material corresponding to the type of component is provided beforehand, the method comprising:
a first step of applying a basic joining material to the circuit board all at one time, the basic joining material commonly usable for joining all of the types of components to the circuit board; and
a second step of applying an individual joining material to locations on the circuit board, to which locations components of a particular type are to be mounted, the individual joining material matching properties of the joining material provided beforehand to the components of the particular type.
According to the present invention, for example, a basic joining material which is commonly usable for all of the types of components to be mounted is applied to a circuit board all at one time. Further, an individual joining material matching properties of a joining material provided beforehand to the components of a particular type is applied. Thus, a circuit board applied with optimum joining materials matching respective joining materials provided beforehand to terminals of a plurality of types of components can be manufactured.
The above-mentioned first step and second step of applying joining materials are interchangeable in sequence. That is, the second step may be performed before the first step. Additionally, the second step may be performed several times to apply different kinds of individual joining materials respectively to a plurality of different types of components. It should be noted that the kind of joining material provided beforehand to the terminals of the components and the kind of joining material to be used in mounting do not necessarily have to be the same. If a basic joining material can adequately join components and a circuit board, those components do not particularly require individual joining materials to be applied.
In the above-mentioned second step of applying individual joining materials, it is preferable that the above-mentioned individual joining materials are selected so as to provide the same composition as the joining material provided beforehand to the components. Accordingly, when combined together, the basic joining material and the individual joining materials have the same composition as the joining material provided beforehand to the terminals of the components, thus increasing the wettability and adhesion.
Metallic materials commonly usable for joining all of the components can be used as the above-mentioned basic joining material, such as paste consisting solely of tin and paste consisting of a mixture of tin and silver. On the other hand, the individual joining materials may include kinds of material provided beforehand to the components. The individual joining materials, for example, may include at least one selected from a group consisting of bismuth, silver, copper, zinc and indium. Kinds of material which might possibly cause mismatching problems when combined with other kinds of solder may be included in the individual joining materials, so that conventional mismatching problems do not occur between solder of different kinds. Additionally, some material preferred to be added to solder of a certain kind may be included in the individual joining materials.
In the above-mentioned first step and second step of applying joining materials, joining materials may be applied to the above-mentioned circuit board either in the form of paste or by means of a plating method.
Additionally, in order to achieve the above-mentioned objects, there is provided according to another aspect of the present invention a method of mounting on a circuit board a plurality of types of components having terminals on which a joining material corresponding to the type of component is provided beforehand, the method comprising:
a first step of applying a basic joining material to the circuit board all at one time, the basic joining material commonly usable for joining all of the types of components to the circuit board;
a second step of applying an individual joining material to locations on the circuit board, to which locations components of a particular type are to be mounted, the individual joining material matching properties of the joining material provided beforehand to the components of the particular type; and
a third step of heating to join the circuit board and the plurality of types of components after completion of the first step and the second step.
Additionally, in order to achieve the above-mentioned objects, there is provided according to another aspect of the present invention a device for manufacturing a circuit board on which are to be mounted a plurality of types of components having terminals on which a joining material corresponding to the type of component is provided beforehand, the device comprising:
first means for applying a basic joining material to the circuit board all at one time, the basic joining material commonly usable for joining all of the types of components to the circuit board; and
second means for respectively applying an individual joining material to locations on the circuit board, to which locations components of a particular type are to be mounted, the individual joining material matching properties of the joining material provided beforehand to the components of the particular type.
The above-mentioned first and second means for applying joining materials are also achieved by application of a printing method and a dispensing method.
The above-mentioned device for manufacturing a circuit board is achieved by a printing method,
wherein the first means has a blanket solder mask provided with openings to apply the basic joining material to all locations on the circuit board at one time, at which locations the components are to be mounted, the basic joining material in the form of paste; and
the second means has a sectional solder mask provided with openings to apply the individual joining material to predetermined locations on the circuit board, at which predetermined locations the components of the particular type are to be mounted, the individual joining material in the form of paste,
wherein, by the first means and the second means, lamination of the basic joining material and the individual joining material is formed at the predetermined locations on the circuit board.
Additionally, the above-mentioned device for manufacturing a circuit board can be more productive,
wherein, when the blanket solder mask is arranged facing the circuit board, the sectional solder mask is arranged over the blanket solder mask so that the lamination of joining materials is formed by consecutively applying the basic joining material and the individual joining materials.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.